Autodeposition has been in commercial use on steel for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. No. 3,592,699 (Steinbrecher et al.); U.S. Pat. Nos. 4,108,817 and 4,178,400 (both to Lochel); U.S. Pat. No. 4,180,603 (Howell. Jr.); U.S. Pat. Nos. 4,242,379 and 4,243,704 (both to Hall et al.); U.S. Pat. No. 4,289,826 (Howell, Jr.); and U.S. Pat. No. 5,342,694 (Ahmed) as well as U.S. Pat. No. 5,500,460 (Ahmed et al.) and U.S. Pat. No. 6,645,633 (Weller et al.). The disclosures of all of these patents are hereby incorporated by reference.
Autodeposition compositions are usually in the form of liquid, usually aqueous, solutions, emulsions or dispersions in which active metal surfaces of inserted objects are coated with an adherent resin or polymer film that increases in thickness the longer the metal object remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any resin or polymer, in the absence of contact with active metal. “Active metal” is defined as metal that is more active than hydrogen in the electromotive series, i.e., that spontaneously begins to dissolve at a substantial rate (with accompanying evolution of hydrogen gas) when introduced into the liquid solution, emulsion or dispersion. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods, or a like term. Autodeposition is often contrasted with electrodeposition, which can produce very similar adherent films but requires that metal or other objects to be coated be connected to a source of direct current electricity for coating to occur. No such external electric current is used in autodeposition, instead an accelerator is used.
The autodeposition accelerator component is a substance such as an acid, oxidizing agent, and/or complexing agent capable of causing the dissolution of active metals from active metal surfaces in contact with the autodeposition composition thereby driving the coating deposition. The autodeposition accelerator component can be chosen from the group consisting of hydrofluoric acid and its salts, fluosilicic acid and its salts, fluotitanic acid and its salts, ferric ions, acetic acid, phosphoric acid, sulfuric acid, nitric acid, hydrogen peroxide, peroxy acids, citric acid and its salts, and tartaric acid and its salts. The autodeposition accelerator component may be selected from any material or combination of materials known for this purpose in prior autodeposition art or otherwise found to give satisfactory results.
Autodeposition compositions typically may also contain one or more additional ingredients. Such additional ingredients may include surfactants (emulsifying or dispersing agents), fillers, biocides, foam control agents, flow control (leveling) agents, and/or carbon black pigments.
Autodeposition coatings, in the absence of pigment, tend to be colorless or slightly yellow to green, and do not provide adequate hiding power for many commercial uses. Adding pigment is a conventional way to increase hiding power of coatings. Introducing pigment into autodeposition baths has proven to be problematic due to the strongly acidic nature of the baths, which have a pH ranging from 1.0 to 4.0. Previously, autodeposition coatings have been limited to black color, using so called “carbon black” pigments that were stable in acid and dispersible in the working bath.
Conventional pigments are adapted for use in paints, which typically have a pH ranging from 5.5 to 10. The significant difference in pH between paint and autodeposition baths has limited the pigments that can be used in autodeposition baths due to the lack of pigments that are predictably stable in acidic autodeposition baths. Attempts to introduce a non-carbon black pigment into autodeposition to produce a coating in colors other than black have up to now been unsuccessful due to unpredictable behaviors of various pigments, including dissolving into the bath, failing to deposit on the active metal substrate with the polymer, developing coatings that rinsed off of the active metal substrate, and reaction in the bath with other components resulting in “crashing” of the bath, as well as settling out of dispersion to form sludge on the tank bottom.
A particular problem in formulating a white or off-white autodeposition coating has been the limited stability of pigments in the autodeposition bath, which typically is subject to the periodic addition of oxidizing agents and contains strong acid. In particular, titanium dioxide (TiO2), an economical and commonly used white pigment, is unstable in autodeposition bath chemistry due to the presence of hydrogen fluoride (HF). In the presence of HF, TiO2 hydrolyzes to fluorotitanic acid and the bath becomes unstable. These changes to the titanium dioxide make it unavailable for deposition on metal substrates as a white pigment for generating white, off-white or gray coatings. Another drawback of prior attempts to use titanium dioxide particles alone has been that the particles do not remain dispersed and form sludge which requires disposal.